The invention relates to an apparatus in accordance with the general part of Claim 1.
In the operation of dryers with hot gas, efforts have been made for some time to recover the heat contained in the exhaust gases of the dryer by passing them through one or more indirectly acting heat exchangers whereby a second fluid medium is heated. In many cases it is unavoidable that the hot gas flowing through the dryer picks up contaminants. In the drying of wood chips, for example, wood dust and natural and artificial resins as well as terpenes are picked up, which condense and precipitate in the heat exchangers due the cooling of the exhaust gases.
Through the article, "Waermerueckgewinning in industriellen Trockneranlagen" [heat recovery in industrial dryers] by H. Hausmann, printed in the "Haus der Technik" lecture text, Essen, May 30-31, 1978, heat exchangers of a modular or drawer type became known, from which small, drawer-like exchanger units can be temporarily withdrawn and can be cleaned individually in baths by means of a washing liquid. The periodical manipulation and cleaning of the drawers must be performed by hand or by an expensive apparatus, and requires, as a rule, the presence of several workers. Furthermore, to forestall the fouling of a heat exchanger, it is also known to remove impurities from the exhaust air with great effort and at great expense by means of active carbon filters, thermal afterburning, electrical filters, and fiber mat filters.
German Offenlegungsschrift No. 30 09 938 discloses an apparatus having a dryer for organic substances in accordance with the general part of Claim 1, in which, for the cleaning of a heat exchanger, a heating gas circuit is intermittently connectable to the heat exchanger in place of the exhaust gas circuit, the heating gas circuit containing a collector means in the form of filters which is separate from the first heat exchanger, and these filters are preferably combustible so that, after they have become charged with harmful substances from the cleaning gas, they can be removed and crushed and used for heating purposes. In this apparatus, several heat exchange units can be provided in parallel or in series, and exhaust gas or heating gas is fed to them in an alternating cycle.
The use of a collector means having a number of filters which have to be periodically replaced, in conjunction with an additional burner for the production of the hot gas in the heating gas circuit, appears to be relatively expensive and complex.
It is the object of the invention to make it possible, in an apparatus of the kind described, to clean fouled heat exchangers without the need for a filtering system and a separate burner.